Prenatal Exposure to Alcohol: What the Images Reveal.

Case studies and statistical analyses of groups of children with fetal alcohol syndrome (FAS) or prenatal exposure to alcohol (PEA) have been performed using magnetic resonance imaging (MRI) techniques. These studies show that alcohol exposure during pregnancy can result in a range of structural brain abnormalities. Among severely affected children, widespread damage may occur to the corpus callosum, basal ganglia, hippocampus, and cerebellum. Throughout the range of severity, however, overall reduction of the brain is apparent, and in particular, portions of the cerebellum, basal ganglia, and corpus callosum are disproportionately reduced in size among children with FAS and PEA. These children also have been shown to have cognitive deficits, although these have not been linked to abnormalities in specific brain areas.

T he effects of exposure to alcohol is made when an alcoholexposed infant deficits and behavioral difficulties related to during the prenatal period have been exhibits pre or postnatal growth deficiency, prenatal alcohol exposure can be apparent studied intensively using a variety of a specific pattern of anomalies in the struc even at the mild end of the continuum (see research methods, including autopsies, ture of the face and skull (i.e., craniofacial Jacobson and Jacobson 1994). For example, animal models, and imaging techniques. anomalies), and some central nervous sys researchers have reported that exposure to Through these efforts, investigators have tem dysfunction (see figure 1). In addition even small amounts of alcohol (1-2 drinks learned that prenatal alcohol exposure 1 can to causing FAS, prenatal alcohol exposure per day) during prenatal development can have devastating effects on the developing can result in a variety of other severe effects, lead to impaired attention (for review, see fetus and that these effects can be viewed as such as mental retardation and additional Streissguth 1986). The terms "fetal alcohol falling along a continuum. At the farthest physical anomalies. But lesser cognitive effects" (FAE) and "prenatal exposure to al end of the continuum are perinatal death cohol" (PEA) are sometimes used to describe and fetal alcohol syndrome (FAS) (see people who do not exhibit all the features SARAH N. MATTSON, PH.D., is associate Jones and Smith 1973). An FAS diagnosis required for a diagnosis of FAS but for director and EDWARD P. RILEY, PH.D., whom some physical or behavioral problems is director of the Center for Behavioral The research discussed in this paper was dren prenatally exposed to alcohol, no one amount probably varies from one motherfetus pair to another, the National Institute on Alcohol Abuse supported by National Institute on Alco has yet defined precisely what brain struc to apply imaging techniques-such as magnetic resonance imaging (MRI), which produces pictures of the brain's structureto investigate this question. MRI studies have shown that structures such as the cerebral cortex, cerebellum, basal ganglia, hippocampus, and corpus callosum appear to be affected by PEA. The cerebral cor tex is the outer part of the brain and is necessary for a variety of higher cognitive functions; the cerebellum is found at the back of the brain and is involved in motor as well as cognitive activities; the basal ganglia are a group of structures that lie deep within the brain (under the cortex) and are involved in motor and cognitive skills. The hippocampus is involved in memory, and the corpus callosum is the tract of nerve fibers bridging the two brain hemispheres. 2 This article reviews MRI findings that reveal a pattern of structural anomalies in the brains of children with FAS or PEA. Such research eventually may be linked to specific developmental and behavioral deficits observed in these children to clarify the nature of this disorder and discern the true capabilities of those who suffer from it.

Autopsy Reports of Children With FAS
The effects of prenatal alcohol exposure on the developing brain have received a great deal of attention since alcohol was first implicated in causing damage to the unborn fetus (i.e., it is a teratogen) more than 20 years ago. In 1973, when the first cases of FAS were documented, an autopsy was reported detailing severe and wide spread brain abnormalities. These anomalies included failure of the young nerve cells (i.e., neurons) to migrate during develop ment to their appropriate locations in the brain, absence (i.e., agenesis) of the corpus callosum, and general disorganization of the brain tissue. Since that initial case, ad ditional autopsy reports on severely affected children have documented a variety of brain abnormalities (Clarren 1986). Among these is the abnormal development of specific brain areas, such as the cerebellum, corpus callosum, and basal ganglia. Other anomalies associated with alcohol include an increase of fluid in the brain (i.e., hydrocephaly), abnormally small brain size (i.e., micro cephaly), and a complete absence of higher brain structures (i.e., anencephaly). (A re view of the brain damage associated with prenatal alcohol exposure can be found in Mattson and Riley in press).

Animal Studies
In addition to autopsy reports, other infor mation about alcohol's effects on the devel oping brain is available from research using animal models. For example, in mice, pre natal alcohol exposure has been associated with a developmental disorder called holo prosencephaly, which has a range, or spec trum, of severities. In the most severe form of holoprosencephaly, the two hemispheres of the brain fail to separate, signifying a severe defect in brain development. In milder cases, at least a partial separation of the two hemispheres occurs, although mid line structures, such as the corpus callosum, may still be abnormal. In rats, abnormal development of the hippocampus, cerebel lum, and basal ganglia also have been noted following perinatal alcohol exposure. (For a review of the effects of alcohol on brain development in animals, see Miller 1992).

MAGNETIC RESONANCE IMAGING OF CHILDREN WITH FAS
One difficulty in using autopsy reports to generalize the type and extent of brain damage resulting from prenatal alcohol exposure is that most of these children died because of the extent of their alcohol related physical problems and, therefore, are unlikely to convey an accurate picture of alcohol's teratogenicity across the con tinuum. Instead, the autopsy cases likely demonstrate the most extreme effects that alcohol can have on the developing brainthat is, profound disorganization that may result in death. Although the information obtained from these autopsies remains ex tremely valuable in the study of FAS, a few recent reports that make use of brain imaging techniques to evaluate children who more closely represent most of the FAS and FAE or PEA cases appear to take knowledge of the full range of alcohol related brain anomalies a step further.
Over the past 4 years, MRI has been used with some success in FAS and PEA children. MRI is a noninvasive procedure in which a powerful magnet is used to align the atoms in the brain. These atoms absorb energy when an electromagnetic signal is passed through the magnetic field. When the electromagnetic signal is turned off, the atoms "relax" and release energy. The measurement of this energy is used to create pictures of the brain.
Because different types of brain tissue absorb and release different amounts of energy, the tissues appear distinct on the resulting magnetic resonance image (for further detail, see the article by Doria,.
This procedure has proven valuable in the study of prenatal alcohol exposure be cause it allows a clear and detailed picture of the brain without any dangerous or pain ful procedures. Upon clinical inspection, one can see obvious anomalies, such as erroneous neuron migration or an absence or thinning of portions of the corpus callosum. With further computer analysis, the brain can be visually dissected and the areas or volumes of various brain structures quan tified. By comparing the sizes of various brain structures in alcoholexposed chil dren versus nonexposed control children, one can determine which parts of the brain are more affected than others by prenatal alcohol exposure.

Case Studies
To date, only a few MRI studies of children with PEA or FAS have been reported in the literature; these include both case studies and group studies. Case studies typically review the findings from one or several cases and describe the children's demo graphic and physical characteristics in detail. Usually these studies provide de scriptions of the clinical findings (e.g., agenesis of the corpus callosum) in each case. In contrast, group studies use more detailed computer analyses to evaluate the MRI's. In these studies, researchers compare the combined data from a group of alcohol exposed children with data from a group of nonexposed children, trying to determine statistically whether any significant differ ences exist between the two samples.
At this time, six case studies in children with FAS or PEA have included results from magnetic resonance images. These reports primarily have included clinical analyses of the MRI data, such as descrip tions of brain damage consistent with the spectrum of holoprosencephaly, and meas urements of reductions in the size of specific brain areas. The case studies suggest that the brain areas particularly affected by al cohol are the cerebral cortex, cerebellum, basal ganglia, hippocampus, and corpus callosum. These areas comprise much of the total brain, but as discussed below, selected areas appear more affected than others by PEA. Case Reports of Severely Affected Children. The first study using MRI to evaluate a child with FAS was included in a report on children with "malformative" syndromes and mental retardation (Gabrielli et al. 1990). Only one child with FAS was included, and only minimal information was provided. The child's brain was de scribed as displaying relatively diffuse brain damage. Shortly after this initial re port, three children of alcoholic women were examined (Ronen and Andrews 1991). The brains of all three children displayed brain damage along the spectrum of holo prosencephaly. Each child was severely affected, and one died early in life. An additional child with FAS was included in a case report describing two children with corpus callosum abnormalities (Schaefer et al. 1991). This child also was severely affected and died at 6.5 months of age. An MRI prior to her death and a subsequent autopsy (Coulter et al. 1993) revealed se vere and widespread brain damage, includ ing abnormalities of the corpus callosum, basal ganglia, diencephalon, hippocampus, and cerebellum. Again, these case reports are valuable, but like the autopsy reports, they probably represent the more severe end of the spectrum of alcohol's effects. In spite of this bias, a common thread may run through these reports: a pattern of damage that conforms to the spectrum of holoprosencephaly. Whether this is a true pattern or simply an artifact of the global and diffuse brain damage resulting from FAS remains to be determined, pos sibly by comparing FAS children with groups of children who have similar levels of brain damage unrelated to alcohol expo sure (e.g., children with Down syndrome).

Reports on Higher Functioning Children.
In addition to the clinical case reports of severely affected children with FAS, a few case reports of higher functioning children have been documented. The MRI's of two adolescent children with FAS were de scribed in a case report published in 1992 (Mattson et al. 1992). Although these chil dren were definitely affected by their expo sure to alcohol during gestation, they had survived the perinatal period and were func tioning at the level of moderate mental re tardation (i.e., the two children had IQ's of 51 and less than 41, respectively). 3 The MRI's of these two children were evaluated using a detailed analysis in which the vol umes of specific brain structures were com pared with those of normal control subjects (i.e., a volumetric analysis was conducted). Both children had abnormalities of the corpus callosum; in one, the structure was missing (i.e., complete agenesis), and in the other, the callosal body was thin in certain areas. The MRI's also revealed that the normal fluidfilled spaces in the brain (i.e., the ventricles) were enlarged, and volumes of the total brain, cerebellum, basal ganglia, and diencephalon were reduced. Furthermore, the children with FAS were compared with a small group of children with Down syndrome who resembled the alcoholexposed children in the general level of intellectual functioning and degree of microcephaly. The brains of the FAS children showed reductions in the volumes of the basal ganglia and diencephalon when compared with the brains of the Down syn drome children. This finding suggests that prenatal alcohol exposure may particularly affect the growth of these structures. It also emphasizes that mental retardation and microcephaly (conditions that are present in both FAS and Down syndrome) are not sufficient to cause the pattern of brain damage seen in FAS.
Subsequent to this case report, two additional children were described in the literature. These children were exposed to high levels of alcohol during prenatal development but did not meet the tradi tional criteria for a diagnosis of FAS. The MRI's of these PEA children indicated that the basal ganglia were reduced in volume compared with normal control children, even when brain size was taken into account. The diencephalon also was smaller, but not when viewed as a propor tion of the total brain size.

GROUP STUDIES EVALUATING BRAIN STRUCTURE IN CHILDREN WITH FAS AND PEA
In addition to these case reports, a few studies have evaluated MRI results in groups of children with FAS or PEA. These studies have focused, for the most part, on detailed volumetric analyses of the brain. Thus far, the total brain, cere bellum, cerebellar vermis (which is part of the cerebellum), ventricles, basal gan glia, diencephalon, and corpus callosum have been measured. The findings from these reports are summarized below (Mattson et al. 1992(Mattson et al. , 1994Riley et al. 1995;Sowell et al. 1996).

Effects on Overall Brain Size
Children with FAS typically are micro cephalic. This characteristic has been known since the initial case reports of FAS and is often mentioned as one of the diagnostic criteria. Recent MRI studies have confirmed this finding, revealing that total cranial capacity is reduced in children with FAS and PEA.

Effects on Cerebellar Size
Results of MRI studies have revealed that in addition to reductions in the total vol ume of the brain, the size of the cerebellum also is reduced. Of 10 alcoholexposed chil dren who have been studied thus far, 8 have shown reductions in this brain region. An other study evaluated three regions of the cerebellar vermis in nine children with FAS or PEA (Sowell et al. 1996). In the children with either FAS or PEA, the vermal region located the farthest forward (i.e., the anterior vermis), but not the other regions, was re duced compared with that of normal control children. Studies of rats exposed to alcohol during the neonatal period have shown similar results. Specifically, the anterior vermis was reduced in the alcoholexposed rats as a result of a decrease in the number of large, specialized cerebellar nerve cells called Purkinje's cells West et al. 1990) in this part of the brain. Although the number of cells has not been evaluated in humans, it may be that a simi lar loss of cells accounts for the reductions in the anterior vermis seen in children with FAS or PEA.

Effects on Ventricular Size
In children with FAS or PEA, the ventri cles are enlarged. This increase in size may be related to the reduction in brain size mentioned earlier. In other words, decreases in brain tissue may go handinhand with in creases in fluidfilled spaces. Whether the increase in ventricular volume noted in chil dren with FAS or PEA is caused by the birth of fewer brain cells or an increase in the rate of cell death is not known. However, the volume of fluid that surrounds the brain within the cranial vault also has been found to be increased in a few children with FAS. Because the size of the vault of the cranium is determined, at least in part, by the growth of the brain, additional fluid surrounding the brain suggests increased cell death-that is, the brain grew, determined the size of the cranial vault, then shrank in volume, possi bly as a result of cell death. Alternatively, changes in both cell death and cell birth may contribute to these reductions in brain size in FAS children.

Effects on the Basal Ganglia and Diencephalon
The basal ganglia and diencephalon are groups of structures that lie deep within the brain and, as mentioned earlier, are associated with both cognitive and motor abilities. Initially, children with FAS ap peared to show reductions in both these brain regions. In the first two FAS chil dren evaluated by Mattson and colleagues (1992), both of these brain regions were reduced when compared with those of nor mal children. Later studies revealed that when total brain size was controlled to ac count for the FAS children's microcephaly, the diencephalon was not disproportionate ly smaller in the FAS children than in the control children (Mattson et al. 1994;Mattson and Riley in press). This finding indicates that although the diencephalon is reduced in FAS subjects when compared with control subjects, it is probably a result of the reduction in brain size caused by the alcohol exposure. Therefore, the diencephalon probably is not more sensi tive to prenatal alcohol exposure than the brain as a whole. In contrast, even when total brain size is accounted for, the vol ume of the basal ganglia in FAS subjects is disproportionately reduced when com pared with that of the control subjects. That is, even when the basal ganglia are viewed as a proportion of total brain size or when equally microcephalic control subjects are used (e.g., children with Down syndrome), this group of structures contin ues to be smaller than normal. This result suggests that the basal ganglia may be especially sensitive to the effects of prenatal alcohol exposure (see figure 2). Further studies have revealed that the total reduction in the basal ganglia may be primarily attributable to reductions in the caudate nucleus. Specifically, when the basal ganglia are separated into subsections (i.e., the caudate and lenticular nuclei), the reduction in the caudate nucleus is signifi cant, whereas that of the lenticular nuclei is not. The caudate nucleus is traditionally associated with cognitive functioning, whereas the putamen (part of the lenticular region) may be more important for motor functioning. This result may help explain some deficits suffered by children with FAS or PEA, but further study and repli cation are required before any conclusions can be reached.

Effects on the Corpus Callosum
The corpus callosum allows information to travel between the brain's two hemispheres and permits the two halves to work together. Some of the earliest reports of brain dam age in children with FAS described com plete agenesis of the corpus callosum. This brain abnormality occurs infrequently in the normal population, 4 at a rate of about 1-3 per 1,000 (0.1-0.3 percent) (NIAAA 1990). In the developmentally disabled pop ulation, the incidence increases to about 2.3 percent (Jeret et al. 1986). In FAS and PEA, however, the incidence may be even higher. Out of 44 children in Mattson and colleagues' sample (e.g., Riley et al. 1995), the researchers are aware of 3 children (6.8 percent) who have callosal agenesis (see figure 3).
In addition to reports of corpus callosum agenesis, information also is available on the size of this structure in children with FAS or PEA (Riley et al. 1995). Ten chil dren with FAS or PEA, excluding the chil dren with agenesis, were evaluated using the area of a photographic slice of the cor pus callosum. 5 In these 10 children, the area of the slice was smaller than that of normal control children. Further analyses were conducted by dividing the area of the corpus callosum into five parts. When brain size was accounted for by measuring the callosal area as a proportion of brain area, three of five callosal regions were reduced in size compared with those of control subjects. The most anterior region and the two posterior regions of the corpus callosum were significantly different (i.e., statistically) from that of the control sub jects. Notably, similar findings have been reported in children with attention deficit hyperactivity disorder (Hynd et al. 1991). 6 Given the common occurrence of attention deficits in children with FAS, this overlap may lead to further understanding of the etiology of attentional deficits and callosal abnormalities in both alcoholexposed and nonexposed children.

SIGNIFICANCE OF FINDINGS
As the findings describe above, alcohol can have devastating consequences on the developing fetus. MRI and other research conducted thus far suggest that several parts of the brain are reduced in size following prenatal alcohol exposure. But not all the regions evaluated thus far have shown spe cific size reductions; although the basal ganglia and corpus callosum appear to be disproportionately smaller in alcohol exposed children, the diencephalic struc tures diminish in proportion to the reduction in total brain size. In another component of this pattern, the anterior vermis of the cerebellum appears to be disproportion ately reduced in size relative to other vermal areas.
The full significance of the findings from MRIbased research remains unclear. Although we know that certain brain areassuch as the basal ganglia, cerebellum, and corpus callosum-appear to be especially sensitive to prenatal alcohol exposure, we do not yet know what functional consequences these size reductions may have. Similarly, although other brain areas (e.g., the dien cephalon) are reduced in volume only as much as the brain is reduced in size, these volume reductions also may have func tional consequences. Although not empha sized here, the FAS and PEA children that Mattson and colleagues have evaluated and described certainly have cognitive deficits. For the most part, their cognitive ability falls below the normal range. Chil dren with FAS typically are mildly men tally retarded, with an average IQ of about 70. Ability levels range considerably, how ever, and IQ's have been reported between 20 and 120 (the average IQ for the general population is 100). In addition, specific cognitive deficits have been reported in several domains, including language, at tention, verbal learning, and memory.
The relationship between the reported cognitive deficits and brain abnormalities is not yet known, although some degree of correlation can be assumed. Mental retar dation in itself, however, does not indicate what pattern of brain damage exists. For example, when FAS children were com pared with Down syndrome children, both groups had similar levels of mental retarda tion and microcephaly. However, a different pattern of brain abnormality was observed in each of the two groups. The children with FAS had reductions in the basal ganglia, but the children with Down syndrome showed relatively minimal damage in this brain re gion, even given their microcephaly. These results suggest that mental retardation and microcephaly cannot predict reductions in the basal ganglia and further emphasize the sensitivity of the basal ganglia to prenatal alcohol exposure. The differences in brain development may be related to differences in cognitive strengths and weaknesses exhibited by the two groups.

FUTURE DIRECTIONS
As scientists learn more about the cognitive deficits and brain abnormalities in FAS and PEA, they will need to link these two areas of research. Although relating brain struc ture and function is difficult both in theory and practice, this connection will reveal the most about the brain and how it is affected by alcohol. The brain differences now being discovered between alcoholexposed and normal control children are exciting and interesting. However, until researchers know what these differences mean for in dividual children with FAS, their capacity to provide assistance is limited. An under standing of which brain areas are affected may enable researchers in the field to develop hypotheses about brain chemistry and suggest pharmacological treatments for children with FAS. MRI research may help solve questions that arise even from children with mild cognitive deficits re sulting from minimal alcohol exposure during pregnancy. The field of brain chem istry research also requires further study. Imaging techniques, such as functional MRI or positron emission tomographywhich evaluate brain activity as people perform tasks (for further details, see the article by Doria, pp. 261-265)-may soon make it possible to determine what the neurochemical parameters are in children with FAS. Neuropsychological assessment is another valuable tool in evaluating alco hol's teratogenic effects. By testing differ ent skills, such as learning and memory, neuropsychology allows both practical and theoretical understanding of the real abili ties and disabilities of children with FAS and PEA. Such knowledge may lead to the design of remediation strategies useful to teachers and other caregivers. In addition, MRI may ultimately become useful as a diagnostic tool among prenatally exposed children in whom the facial features signaling FAS are absent.

SUMMARY
FAS is a devastating disorder that affects children prenatally exposed to high levels of alcohol. This alcohol exposure causes abnormalities in a child's physical as well as psychological wellbeing. Specifically, the developing brain appears to be espe cially sensitive to prenatal alcohol expo sure, the effects of which are permanent. Regions that may show increased sensitivity to this exposure include the cerebellum, corpus callosum, and basal ganglia. The functional consequences of this pattern of brain damage have yet to be uncovered, although a fair amount is known about the cognitive abilities of children with FAS. Future research will continue to focus on uncovering the brain abnormalities resulting from prenatal alcohol exposure and deter mining the relationships between these abnormalities and everyday functioning. By understanding these relationships, re searchers hope to further knowledge of how alcohol's effects can be prevented and treated.